1. Field of the Invention
The present invention relates to a traction control system for controlling a braking force applied to each driven wheel of an automotive vehicle having four driven wheels, and more particularly to the control system which prevents each wheel from slipping which will occur during, for example, accelerating the vehicle, by applying a braking force to each wheel of the vehicle in accordance with the slipping condition of the wheel.
2. Description of the Related Arts
An ordinary passenger vehicle has a pair of wheels at each of its front and rear sides. Either the front wheels or the rear wheels of that vehicle are operatively connected with an engine to be driven directly thereby, while the rest of the wheels are not connected with the engine so as to be served as non-driven wheels. A vehicle having the driven wheels at its front side is called a front drive vehicle, while a vehicle having the driven wheels at its rear side is called a rear drive vehicle. Whereas, a vehicle having the driven wheels at both of the front and rear sides is called a four-wheel drive (4WD) vehicle. As for a driving system of the four-wheel drive vehicle, various types of the system are known, such as a part time system, full time system, and the like. According to the full time system, the front driven wheels and rear driven wheels are connected by a differential gear mechanism, i.e., so called a center differential gear.
Also, in order to prevent an acceleration slip from occurring in the case where an excessive driving force is applied when starting or accelerating the vehicle, an acceleration slip control system, i.e., the traction control system is getting popular in the market, as disclosed in a Japanese Patent Laid-open Publication No.8-133054, for example.
In the case where the traction control system is applied to the four-wheel drive vehicle, however, there may be such a case that hydraulic braking pressure is supplied from an auxiliary pressure source to each wheel brake cylinder, with the communication between a master cylinder and all of the wheel brake cylinders blocked, and controlled in response to the slip condition of the wheel. This is the case where all of the wheels are under the acceleration slip, for example. In this case, however, when a vehicle driver depresses a brake pedal and an accelerator pedal simultaneously, or when the driver depresses the brake pedal immediately after he released the accelerator pedal, it will be difficult for him to obtain an expected stroke of the brake pedal, so that a feeling in his braking operation will be deteriorated. Furthermore, if the hydraulic braking pressure in the wheel brake cylinder was relatively high, when the traction control was terminated, that hydraulic braking pressure would be applied to the master cylinder immediately after the termination of the traction control. Therefore, some components of the master cylinder might be damaged, if an excessive load was applied thereto by the hydraulic braking pressure returned from the wheel brake cylinder.